When a ray of light passing through air strikes a plane surface of a transparent medium, part of the ray is refracted into the transparent medium. The incident reflected and refracted rays all lie in a common plane which is perpendicular to the surface of the medium in question. The normal is defined as a line perpendicular to the surface at a point where the ray strikes a transparent medium.
As measured from the normal, the angle of reflection equals the angle of incidence. The angle of refraction, as measured from the normal, is defined by Snell's law as sin I/sin R=U where I is the angle of incidence, R is the angle of refraction, and U is the index of refraction of the medium in question. Except for the rays perpendicular to the surface, the angle of incidence is always greater than the angle of refraction.
For ordinary glass, which has a refractive index of about 1.5, a ray at grazing incidence is refracted into the glass at an angle of about 40 degrees. Reciprocally, a ray passing through the glass at an angle of 40 degrees from normal will emerge into air at grazing incidence to the surface of the glass; this angle is termed the critical angle for glass or any other transparent medium having the same refractive index.
If a ray passing through the medium in question should strike the surface at an angle greater than 40 degrees with respect to the normal it is obvious that the laws of refraction can no longer obtain, and the ray is totally reflected from the surface back into the medium. Depending upon its refractive index, there is a critical angle for every transparent medium; rays originating within the medium and striking a plane surface at an angle with respect to the normal greater than the critical angle are totally reflected within a medium.
The present invention utilizes the phenomenon of total internal reflection within parts of cones and cylinders, having complementary conical cavities, to achieve an even distribution of light over a surface to be illuminated and to capture certain transient light rays and divert the same from the line of sight of an observer of a surface so illuminated.
The most pertinent prior art to the present invention is U.S. Pat. No. 3,246,133, issued to the present inventor, and the disclosure of that patent is specifically incorporated herein by reference.
As set out in the prior patent, it is essential that an aircraft pilot have visual access to the dial faces of various meters and navigational instruments. However, an excess of transient light within the cockpit leads to a phenomenon commonly known as night blindness which is essentially the reduction of visual acuity for distant or faintly lighted objects external to the aircraft.
In addition to the obvious need for glare suppression, the problem involves the efficient distribution of illuminating light over the several dial faces so that information bearing rays may be provided with uniform intensity and without extraneous light which serves no useful function.
Part of the difficulty resides in the inverse square law of illumination from a point source. Without proper corrective measures as set forth herein, a light bulb positioned out of a pilot's line of sight above an instrument dial to achieve an indirect lighting effect tends to illuminate those surfaces nearest the bulb to a greater extent than more remote areas because the illumination thus provided is approximately inversely proportional to the square of the distance from the filament of the bulb. Consequently, uniform lighting can only be achieved by this method when the illuminated area is small in comparison to the distance from the bulb.
The illumination system disclosed in the prior art patent herein referenced has largely solved the foregoing problems and has been successfully utilized in many aircraft for many years. However, a problem in even light distribution has become manifest which will be described and which has been solved by the present invention.